Chromium-nickel-aluminum steel and method



United States Patent 3,278,298 CHROMIUM-NICKEL-ALUMINUM STEEL AND METHODD Cameron Perry, Middletown, Ohio, assignor to Armco Steel Corporation,Middletown, Ohio, a corporation of Ohio No Drawing. Filed Dec. 31, 1963,Ser. No. 334,923 Claims. (Cl. 75128) My application for patent is acontinuation-in-part of my copending application, Serial No. 227,731,filed October 2, 1962, and entitled Chromium-Nickel-Aluminum Steel andMethod, now abandoned. The invention is concerned with theprecipitation-hardenable stainless steels, more particularly thechromium-nickel-aluminum steels and the heat-treatment of the same.

One of the objects of the invention is the provision of asemi-austenitic chromium-nickel-aluminum stainless steel which may beshipped from the mill in severly cold weather without risk of prematurehardening; which is readily formed or fabricated in one condition ofheattreatment; and which is strong and yet tough and ductile in anothercondition of heat-treatment.

Another object of my invention is the provision of aprecipitation-hardenable chromium-nickel-aluminum stainless steel whichreadily lends itself to hot-working in the mill as in the production ofvarious flat-rolled products such as plate, sheet and strip, as well asin the production of bars, rods, wire and the like; and, which, inaddition, is characterized by good cold-working properties as in theproduction of cold-rolled plate, sheet, strip and the like and in theproduction of cold-drawn rods and wire, and other products which arecharacterized by strength, toughness and ductility not only in thelongitudinal direction but in the transverse and short transversedirections, as well.

A further object is the provision of a simple, direct, and effectivemethod for heat-treating a corrosion-resistant chromium-nickel aluminumsteel of the character desribed to develop optimum mechanical propertiesin the precipitation-hardened condition, more especially hightensilestrength and high-yield strength in combination with good ductility andtoughness, particularly sharpnotched toughness as evidenced by greatresistance to crack propagation.

A still further object is the provision of particular articles ofmanufacture fashioned of the steel and the flatrolled products of myinvention in Which there is had a high strength-to-weight ratio;notably, air frames, skin and other parts of supersonic aircraft,advanced missile motor casings, rocket casings and other pressurevessels where stresses are encountered along all three axes.

An additional object is the production of precipitationhardenable,corrosion-resistant chromium-nickel-aluminum steel castings, and variousprecipitation-hardened cast articles and products fashioned of the same,in which strength, durability and toughness are had.

Other objects of my invention in part will be obvious and in part willbe pointed out during the course of the description which follows.

The invention, accordingly, consists in the chemistry of the steel interms of the combination of ingredients employed and in the correlationof the same, in the temperature and cycles of heat-treatment employed,and in the articles and products fashioned of the steel, all as moreparticularly described herein, the scope of the application of which ismore particularly set forth in the claims at the end of thisspecification.

As an aid to a better understanding of certain aspects of my invention,it may be noted at this point that a variety of precipitation-hardenablegrades of stainless steel are 3,278,208 Patented Oct. 11, 1966 widelyaccepted in the art. Among the grades favorably accepted are thechromium-nickel-titanium steels. Others are the chromium-nickel-copperstainless steels. Still others are the precipitation-hardenablechromium-nickel-aluminum steels.

While the several precipitation-hardenable stainless steels noted abovepossess many desirable characteristics, depending upon specific analysisand heat-treatment, those characteristics of necessity are accompaniedby others of less desirability. A case in point is theprecipitation-hard ena'ble chromium-nickel-titanium steel. These steelsincline to be dirty as a result of the titanium content and the easewith which titanium nitrides are formed in melting and in teeming; themechanical properties are inclined to suffer. Similarly, thechromium-nickeLcopper stainless steels, while readily workable in onecondition of heat-treatment, do not reach the hardness and strengthnecessary for many applications when subjected toprecipitation-hardening treatment. And the knownchromium-nickel-aluminum stainless steels in many instances, paricularlyin fiat-rolled products, are somewhat lacking in toughness, especiallywhere great stresses are encountered in the transverse direction.

It is the chromium-nickel-aluminum grades which are of particularinterest here in that they possess a surprising combination of goodforming and fabricating properties in the annealed or solution-treatedcondition of the metal; are readily hardened by heat-treatment atmoderate temperatures; and in age-hardened or precipitation-hardenedcondition possess great strength and hardness. In this connection,attention is invited to US. Patents 2,505,- 762; 2,505,763; 2,505,764and 2,506,558; all issued May 2, 1950 to George N. Goller.

Perhaps the greatest mechanical strength had in thechromium-nickel-aluminum grades of precipitation-hardenable steel areachieved in the grade which is modified by the presence of substantialquantities of molybdenum. Unfortunately, this steel lacks the toughnessdesired for many applications. And while toughness may be developed byoverageing the steel, this results in a loss of tensile properties.

Accordingly, among the objects of my invention is the provision of asemi-austenitic chromium-mickel-aluminum stainless steel which isreadily worked at the mill into a variety of flat-rolled and otherproducts, notably plate, sheet, strip, billets, bars, rods and wire;which is of such composition balance that it may be shippedcross-country even in extremely cold weather, with-out prematurelyhardening; which readily lends itself to fabrication as by bending,pressing, shrinking, stretching, cutting, drilling and the like and byriveting, brazing, welding and the like; and which steel and theproducts fashioned thereof may be hardened by a precipitationheat-treatment at moderate temperatures to a desired combination ofmechanical properties, particularly high strength together with goodductility .and toughness, and even sharp-notched toughness.

Turning now to the practice of my invention, 1 provide a semi-austeniticchromium-nickel-aluminum stainless steel of especially low carboncontent, especially low sulphur content and especially low nitrogencontent. The steel of my invention essentially consists of carbon notexceeding 0.05% (preferably in the amount of 0.002% to 0.050% fordesired strength and toughness as noted hereinafter), chromium in theamount of about 7.0% to 18.0%, nickel in the amount of about 6.0% to12.0%, aluminum in the amount of about 0.5% to 2.5%, with a nitrogencontent which must not exceed 0.05% (preferably not exceeding .03% forassured cleanliness, and especially not over 01% for best impactresistance and toughness) and a sulphur content which must not exceed0.015% (preferably not exceeding 0.010% and most preferably notexceeding 0.005% for best results), and remainder essentially iron.

By remainder essentially iron it, of course, is meant that the remainderof the composition is iron together with such incidental ingredients andadditives as not to detract from the new and surprising properties hadin my steel. For example, manganese is present in the steel, thisamounting to as much as 1.0%, although preferably being maintained at avalue not exceeding 0.50% and most preferably not exceeding 0.1%.Similarly, silicon is present, this in amounts up to 1.0%, althoughpreferably not exceednig 0.50% and most preferably not exceeding 0.1%.The steel additionally may contain molybdenum in an amount up to about8.0%, titanium in an amount up to 0.10%, and boron in an amount up to0.003%. Where desired, small amounts of tungsten, vanadium, zirconiumand columbium may be employed in my steel. Cobalt may be substituted fornickel, this up to about one-half of the total nickel requirement, inthe ratio of three parts cobalt for one part of nickel replaced.

Now the particular amounts of chromium, nickel and aluminum employed inthe steel of my invention and the correlation between these ingredientsgenerally may be viewed as critical. For where lesser amounts ofchromium are employed the desired resistance to corrosion is notachieved. And where greater amounts are employed, the structural balanceof the steel is upset, this with a loss of ultimate hardness in theprecipitation-hardened condition. Similarly, where either lesser amountsof nickel are used than the prescribed minimum a tendency towardinstability results and the metal inclines to prematurely harden in coldweather, or where greater amounts are used than the prescribed maximum,the structure of the steel is radically changed with resulting loss ofmechanical properties. While perhaps there is some latitude in thealuminum content of the steel, here again any substantial decrease fromthe prescribed lower limit or any substantial increase above theprescribed upper limit disturbs the structural balance with theresulting undesired change in the overall mechanical properties.

While the chromium, nickel and aluminum contents of my steel may beviewed as generally critical as to the amounts employed and as to thecorrelation between the same, it is the carbon, sulphur and nitrogencontents which are particularly critical. The carbon content, as notedabove, must not exceed 0.05%, for -I find that with greater carboncontents, the toughness of the steel de creases to an undesirably lowlevel. And actually, from the standpoint of toughness, the lower carboncontents are more desirable in that with them maximum toughness is had.For example, I find that even with as low a carbon content as 0.002%, -Iobtain a steel which is stable against cold transformation in theannealed condition, but can be heat-treated to high strengths andtoughness. This steel, however, requires some 50 hours at theconditioning temperature, prior to refrigeration andprecipitation-hardening, to develop these desirable properties. With asomewhat higher carbon content a shorter time is required forconditioning the metal. For example, at a carbon level of about 02%, theconditioning treatment must be of the order of several hours, and atcarbon contents of about 025% to 0.045%, the conditioning treatment isreduced to about one hour, this prior to refrigeration andprecipitation-hardening, to develop the desired strength and toughness.

While I do not care to be bound by the explanation, my views on theeffect of carbon are as follows. The steels of my invention are stableagainst cold transformation in the solution-treated or annealedcondition with all of the carbon being in solid solution in theaustenite. Upon heating to the austenite conditioning temperature, aprecipitate of carbides probably of the M C type (metal carbides),occurs in the phase boundaries. -In addition, there also is probablyprecipitated, especially in the very low carbon steels, an intermetalliccompound, probably Ni Al At the higher carbon levels, these precipitatesform in the phase boundaries much more rapidly than they do at the lowercarbon levels. These precipitates, in removing from solid solutionpowerful austenite stabilizers such as carbon and nickel, unbalance theaustenite so that upon cooling to refrigeration temperatures,transformation occurs. The precipitation-hardening heattreatment thenfollows to develop the desired strength and toughness.

As pointed out above, the precipitates noted normally occur in the phaseboundaries as continuous networks. These precipitates are hard andbrittle. By reducing the amount of these precipitates, and byeliminating the network, both accomplished by keeping the carbon below.05 the high toughness of these steels is obtained.

The sulphur content of my steel should not exceed 0.015%, as notedabove, and preferably should not exceed 0.010%, is also noted, for thereason that this ingredient seems to appear as an interstitial in thecrystal lattice of the metal. It causes dislocations. And while I do notcare to be bound by the explanation, it is my view that by minimizingthis interstitial there is had a critical reduction in the number ofdislocations in the lattice structure and a resultant increase in thetoughness of the metal. Whatever the explanation, however, I find thatthe sulphur content of the steel must not exceed 0.015% and desirablyshould not exceed 0.010%. Preferably, the sulphur content is maintainedat a value not exceeding about 0.005%.

The nitrogen content of my steel should not exceed 0.05% and preferablyshould not exceed 0.01% in order to enjoy maximum toughness andresistance to impact. I am inclinde to feel that because of the largeamounts of aluminum present, any significant amount of nitrogen givesrise to a strong inclination to form aluminum nitrides, these nitridesbecoming dispersed throughout the steel with a sacrifice of the Weldingproperties of the steel as a result of the nitrides decomposing underthe heat of the welding operation, causing porisity and possibility offailure under load. While soundness and freedom from porosity may beassured through the addition of titanium, I am generally disinclined toemploy more than 0.10% of this ingredient, even though a small excesswould not be harmful, since such an addition is but a corrective measurewhich is inclinded to give dirty metal with some loss of toughness. Bestresults are had by maintaining the low nitrogen content, nitrogenpreferably not exceeding 0.1% and certainly not exceeding 0.05

The ingredient boron may bev employed in my steel as noted above.Actually, I prefer to employ this ingredient in an amount of 0.001% to0.002%, in order to secure good hot-rolling and other hot-workingproperties. The amount of boron, however, should not exceed .003%because boron like sulphur discussed above, is inclinde to appear as aninterstitial in the crystal lattice structure, with resultantdislocation and loss of toughness.

Phosphor-us commonly is present in my steel in small amounts. While notcritical, the phosphorus content should not exceed 0.040%.

While, as indicated above, the ingredient molybdenum may be employed asan additional element in the steel of my invention, I actually prefer toemploy this as an essential ingredient in the amount of some 2.0% to6.50%. Molybdenum contributes to the tensile strength of my steel.Surprisingly enough, however, steels with molybdenum contents exceedingthe 6.5% figure do not develop adequate toughness.

One preferred steel according to my invention essentially consists ofcarbon not exceeding 0.050%, chromium about 13.0% to 15.0%, nickel about7.5% to 9.5%, molybdenum about 2.0% to 3.0%, aluminum about 0.75% to1.50%, and remainder essentially iron. Manganese and silicon each arepresent in amounts up to 1.0%. The nitrogen content must not exceed0.05%. Preferably the nitrogen content is maintained at a value notexceeding 0.03% and especially not over 0.01% for best impact resistanceand toughness. The phosphorus content of this preferred steel does notexceed 0.040%, and the sulphur content does not exceed about 0.010%.Sulphur up to an amount of 0.015% is tolerated where the carbon contentis not in excess of 0.03% (with nitrogen not exceeding 0.050%), or wherethe carbon content is up to 0.03% or even up to 0.05% (with the nitrogencontent not exceeding 0.03%, especially not exceeding 0.01%). Titaniummay be present in amounts up to 0.10% and boron in amounts up to 0.003%.This steel enjoys the combination of strength and toughness.

Another and more specific preferred steel essentially consists of carbonabout 0.025% to 0.04%, chromium about 14.2% to 14.7%, nickel about 8.1%to 8.6%, molybdenum about 2.00% to 2.50%, nitrogen up to about .05%,aluminum about 1.05% to 1.30%, and remainder essentially iron. Manganeseand silicon each is present in amounts up to .70%, preferably each inamounts of .20% to .70%. In this preferred steel the phosphorus does notexceed about 0.040% and the sulphus does not exceed about 0.010% andpreferably not over about 0.005%. The nitrogen content does not exceedabout 0.05 and preferably does not exceed 0.01% for the reasons given.Titanium and boron may be present, titanium in amounts up to 0.10% andboron in amounts up to 0.003%.

A further preferred steel according to my invention, this at somewhatdifferent carbon, chornium and nickel balance, essentially consists ofabout 0.02% to 0.03% carbon, about 13.7% to 14.2% chromium, about 8.0%to 8.5% nickel, about 2.0% to 2.5% molybdenum, about 1.05% to 1.30%aluminum, and remainder essentially iron. The manganese contentpreferably does not exceed 0.50%, manganese in the amount of 0.20% to0.50% being preferred. And the silicon content does not exceed 1.0% andpreferably is in the amount of 0.60% to 1.0%. The phosphorus contentdoes not exceed about 0.040%, and the sulphur content does not exceedabout 0.010% and preferably does not exceed about 0.005%. Nitrogen doesnot exceed 0.05%; preferably it does not exceed 0.01%. Here, too, bothtitanium and boron, one in an amount up to 0.10% and the other in anamount up to 0.003%, may be present.

A still further preferred steel, this again at a somewhat differentcarbon, chromium and nickel balance, with a difference in molybdenumcontent too, essentially consists of about 0.02% to 0.03% carbon, about12.0% to 12.5% chromium, about 8.5% to 9.0% nickel, about 4.0% to 4.5%molybdenum, about 1.05% to 1.30% aluminum, with remainder essentiallyiron. The manganese content does not exceed 0.50% and preferably is onthe order of some .20% to .50%, while the silicon does not exceed 1.0%and preferably amounts to 0.60% to 1.0%. The phosphorus content does notexceed about 0.040%, and the sulphur content does not exceed about0.010%; preferably it does not exceed about 0.005%. Here again, nitrogendoes not exceed 0.05% and preferably it does not exceed 0.01%. Bothtitanium and boron may be included in this preferred steel, the one notexceeding 0.10% and the other not exceeding 0.003%.

An additional preferred steel, again at another differentcarbon-chromium-nickel-molybdenum balance, essentially consists of about0.02% to 0.03% carbon, about 10.25% to 10.75% chromium, about 9.0% to9.5% nickel, about 6.0% to 6.5% molybdenum, about 1.05% to 1.30%aluminum, and remainder essentially iron. Again, the manganesepreferably does not exceed 0.50% and usually is in the amount of .20% to.50%. The silicon content should not exceed 1.0% and preferably amountsto .60% to 1.0%. The phosphorus content does not exceed about 0.040%,and the sulphur content of the steel does not exceed about 0.010%;preferably it does not exceed about 0.005%. Nitrogen, where present, isin amount not exceeding 0.05% and preferably not exceeding 0.01%.'I-itanium may be present in the amounts of 0.10%, and boron ispreferably present in amount up to 0.003%.

The steel of my invention conveniently is made in the electric arcfurnace although, where desired, it may be vacuum-melted. In eitherevent the steel in the form of ingots is converted into slabs, bloomsand billets or it may be continuously cast into slabs, blooms andbillets. And upon reheating, hot-rolled into plate, sheet, strip, bars,rods, wire and the like. As previously noted, the metal works well inthe hot mill. Of course, it forges easily.

Now the steel in the form of plate, sheet, strip, bars, rods, wire andthe like, may be supplied various customersfabricators in the annealedcondition, or it may be supplied in the annealed plus box-stabilizedcondition. Where desired, it may be supplied in cold-rolled condition.The composition balance of the steel is such that partial transformatinas a result of cross-country shipping, even in the most severe cold, iseffectively precluded.

In the annealing or solution-treatment, whether performed at the steelmill or in the plant of the customerfabricator, heating at some 1800 F.to 2000 F. is employed. Usually for most fiat-rolled products, and evenfor the other hot-Worked products, solution-treatment at about 1850 F.for a time of 3 minutes for each 0.1" thickness of the metal givessatisfactory results. The annealing or solution-treatment apparentlyplaces the metal in an austenitic condition in which the aluminumcontent of the steel is dissolved. And upon quenching the steel eitherin air, oil or Water, following the anneal ing treatment, the aluminumconstituent remains in solution. The metal is essentially austenitic andis soft, ductile and readily workable.

In the solution-treated condition my steel has a hardness on the orderof about Rockwell B to 90. It readily lends itself to forming andfabrication by cutting, bending, pressing, drilling, and the like, aswell as by machining, tapping and threading. Additionally, the steel maybe brazed and welded, the welding properties being superior to the knownprecipitation-hardenable chromium-nickel-aluminum steels. It is suitedto the production of a variety of supersonic aircraft parts such asframes, skin and the like, and to the production of defense missilemotor cases, rocket cases, pressure vessels and high pressure tankage.Additionally, it is suited to the production of fastening devices suchas bolts, screws, studs and the like, and to the production of fluidvalves, valve seats and valve parts generally.

Following fabrication the steel is given an austeniteconditioningheat-treatment at a temperature of 1300 F. to 1750 F. for a time of onehour or more. The metallurgy of the austenite conditioningheat-treatment is discussed above.

Following the austenite-conditioning treatment, the steel of myinvention is cooled or refrigerated at a temperature between about 60 F.and a low of 200 F. Usually I find refrigeration at a temperature ofsome F. to F. to be eminently satisfactory; ordinarily a temperature of100 F. for about 8 hours effects the desired transformation from anaustenitic condition to a martensitic condition. There is a significantchange in hardness with this treatment. Nevertheless, where desired,limited forming and fabricating operations actually may be conductedwith the steel in the transformed con dition rather than in the annealedcondition. Actually, certain of the machining operations, for example,sawing, drilling, threading and the like, may be best effected with themetal in the transformed condition, taking advantage of the hardeninghad as a result of the transformation treatment.

Final hardening of the steel is had by reheating at a temperature ofsome 700 to 1200 F. and cooling in air,

oil or water. Ordinarily, I treat the steel at a temperature of some 900F. to 1050 F. for several hours, and cool, although for most purposesthe reheating at a temperature of 950 F. for 1 hr. and quenching in air,oil or water, gives excellent results. The hardness had is on the orderof some Rockwell C35 to 50.

Of particular importance, the steel of my invention in the age-hardenedor precipitation-hardened condition possesses an excellent combinationof mechanical strength and toughness. The strength realized is on theorder of some 200,000 p.s.i. to 255,000 p.s.i., with an Allison Bend 1figure indicative of toughness being at least .40 to .50 at an ultimatetensile strength level of 230,000 p.s.i. to 240,- 000 p.s.i. This degreef toughness at these levels of ultimate tensile strength is mostsurprising.

Additionally, the steel in the age-hardened condition is tough andpeculiarly resistant to crack propagation. Center notched, fatiguecracked, specimens, .050" thick by 2.000" wide, with slow crack growthfollowed by ink stain, developed K0 values calculated to be near 230,-000 p.s.i. /i n. The value developed, however, is not the true Kc value;the true Kc value may be greater than this since the net section stressimmediately prior to fracture was greater than the tensile yieldstrength of the steel.

A typical fracture surface on the failed specimen is 100% shear wherethe fracture surface is at approximately a 45-degree angle to the sheetsurface. The fracture surface of a brittle material is typified by acleavage type fracture which is at an angle of about 90 degrees to thesheet surface.

By way of specific illustration of the steels of my invention, thechemical analyses of four of my steels analyzing about 14% to chromium,about 8% to 9% nickel, about 1% aluminum, about 2% to 3% molybdenum,manganese and silicon each not exceeding about .7%, carbon not exceedingabout 04%, phosphorus and sulphur each not exceeding about 008%,nitrogen not exceeding about .04%, and remainder essentially iron, theseto be compared with four others of differing chemical composition, aregiven in Table I(a) below:

and cold-rolling operations. The standard test samples were prepared andgiven a heat treatment consisting of austen-ite-conditioning,refrigeration and hardening. The samples of the various steels weretested for ultimate tensile strength and toughness (Allison Parameter).The results are given below in Table I b) Table 1(b) ULTIMATE TENSILESTRENGTH AND TOUGHNESS OF THE STEELS OF TABLE I(a) IN THE SRH950CONDITION Ult. Tens. 0.2% Y.S. Percent Rockwell Allison 3 Heat Str.p.s.i. p.s.i. E1. in 0. Parameter Hardness 1 Steel of the presentinvention.

2 Condition 1,700 F.1 hour and air cool; Refrigerate at 100 F. for 8hours; Harden at 950 F. for 1 hour and air cool.

3 Transverse direction.

It will not noted that for the two induction melted steels of myinvention (Heats R3819 and R4225) there is had in the age-hardenedcondition ultimate tensile strengths on the order of some 245,000 p.s.i.to 252,000 p.s.i. with toughness as measured by the Allison Bend Test of.58 and .91. About the same level of strength is had with the vacuummelted heat (Heat V88) with toughness, however, according to the AllisonBend Test of 1.38. The are furnace heat (Heat 31562) achieved a strengthof some 232,000 p.s.i. with toughness of .55 Allison.

As contrasted with the illustrative steels of my invention (Heats R3819,R4225, V88 and 31562) with Allison Parameters of .58, .91, 1.38 and .55,the steels of the higher sulphur content, but otherwise of like chemicalanalysis and of about the same strength levels, had Allison Parameter ofonly some .26 to .34, the actual figures for the four samples being .30,.26, .34 and .32 in the order given. And the calculated toughness forHeat Table I (a) CHEMICAL ANALYSES OF 8 CHROMIUM-NICKEL-ALUMINUMSTAINLESS STEELS Heat; 0 M11 P S Si Cr Ni MO Al N l Steels of thepresent invention.

A Bend Test for Toughness by Dean K. Hanink and George It. Sippel, MetalProgress for August 1960, DD. 89-92, Product Engineering Sept. 4, 1961,pp. 62 to 64.

-Report of a special ASTM Committee (American Somety for Testing ofMetals) Fracture Testing of High-strength Sheet Materials, AS'lMBulletin for January 1960, pp. 29 through 40.

31562, using the Center Notch fatigue-crack test (Kc), exceeded 235,000p.s.i. /in. and net fracture stress exceeded 250,000 p.s.i.

As specific illustration of the steels of my invention of extremely lownitrogen content, the chemical analyses of three analyzing about 14% to15 chromium, about 8% to 9% nickel, about 1% aluminum, about 2% to 3%molybdenum, manganese and silicon each not exceeding about .4%, withcarbon not exceeding about .04%, sulphur and phosphorus each notexceeding about .004%, nitrogen not exceeding about .01% and actuallynot exceeding .001%, and remainder essentially iron, these to becompared with a steel of similar chemical composition except fornitrogen content, are set out in Table II(a):

Table II (a) CHEMICAL ANALYSES OF 8 CHROMIUM-NICKEL-ALUMINUM STAINLESSSTEELS Heat C Mn P S Si C1 Ni M A1 I N O i H 3 1 Steels of presentinvention. 2 In parts per million.

Of the eight heats whose chemical analyses are given 15 gen content .091the Allison Parameter only amounts to above, the Heats V87, V88 and V89are melted in the vacuum induction furnace, the Heats R4223, R4224 andR4225, as well as the Heat R4235, presented for comparative purposes,are melted in the air induction furnace, and the Heat 31562 is melted inthe air electric arc furnace.

The mechanical properties of the eight steels of Table II(a) are givenbelow in Table II(b):

20 invention, there are given the chemical analyses of five steels,three at a molybdenum level of about 2% and another two at a molybdenumlevel of about 4%, in the Table III(a) below:

Table III (a) CHEMICAL ANALYSES OF FIVE CHROMIUM-NICKEL-ALUMINUM-MOLYBDENUM STAINLESS STEELS Table II( b) Heat 0 Mn P s 81 Cr N1 M0 Al N35 The five heats identified above were melted in the induction furnacewith resulting ingots forged, then sur- MECHANICAL PROPERTIES OF THESTEELS OF TABLE II(a) IN THE SRH950 CONDITION Ult. Tens. 0.2% Y.S.Percent Rockwell Allison 3 Heat Str. p.s.i. p.s.i. E1. in Parameter 2'Hardness l Steels of the present invention.

2 Condition 1,700 F.1 hour and air cool; Refrigerate at -100 F. for 8hours; Harden at 950 F. for 1 hour and air cool.

3 Transverse direction.

In connection with the mechanical properties given above noteparticularly the increase in the Allison Parameter of the various steelsas the nitrogen content of the steels is lowered. For the steels of thepresent invention, as the nitrogen content is lowered from a value of.033 for Heat 31562 down to a value of .001 for the Heats V87, V88 andV89, the Allison Parameter rises from .55 to a value of some 1.21 to1.46. For the steel of nitrocontent on the order of 2% (R3840, R3843,R3847) have an ultimate tensele strength in the age-hardened conditionof some 239,000 p.s.i. to 244,000 p.s.i., a toughness according to theAllison Parameter of some .48 to .70. The hardnesss of the steel isabout Rockwell C50. In the an nealed condition the steel, of course, issoft and ductile and readily formable with hardness of Rockwell BSD-85The steels with molybdenum content on the order of 4%, withcorrespondingly lower chromium contents than the steels of the 2%molybdenum content pointed to above, While of somewhat greater hardnessand somewhat greater Table III (b) OPERTI S OF THE FIVE STEELS OF TABLEIII(a) BOTH IN THE ANNEALED CONDITION MECHANICAL PR E AND IN THEAGE-HARDENED CONDITION Condition A Condition RH 950 1 Heat a U.T.S. 0.2%Y.S. Percent RB U.T.S. 0.2% Y.S. Percent R Allison p.s.i. p.s.i. E1. in2" p.s.i. p.s.i. El. m 2" Parameter 1 C ndt'on A-Ar'meal 1 850 F. 1minute and air cool. 2 C ndition' RH 950Au stenite-conditi0ning at 1,750F. for 10 minutes and air-cool; refrigerate at F. for 6 to 8 hours;age-harden by reheat at 950 F. for 1 hour and water-quench.

3 Transverse direction.

In the matter of annealing or solution-treating the steel of myinvention, then austenite-conditioning the same, refrigerating, andage-hardening, I find that the matters of refrigeration temperature andspecific temperature of agehardening are not particularly critical. Theparticular temperature at which the steel is previously conditioned,

however, is critically important.

As generally illustrative of the effect of variableaustenite-conditioning and refrigeration treatments, all at constanthardening treatment of 975 F. for 1 hour and air cool, see themechanical properties for the Heat 31562 as -set out in Table IV below,all being taken in transverse direction:

By interpolating the data given in Table IV I find that therefrigeration temperature at which maximum strength is obtained, butwith minimum toughness, varies from -100 F. to 160 F. as theconditioning temperature is varied from 1350 F. to 1750 F. And as to theconditioning temperature, for refrigeration, for example, at 75 F. for 8hrs. and hardening at 975 F. for 1 hour, optimum strength, but withminimum toughness is had by previously conditioning the steel at atemperature on the order of some 1400 F. By conditioning the steel athigher temperatures, there is had a definite increase in the toughnessof the metal although there is a decrease in the strength. And byincreasing the duration of the time at conditioning temperature, thereis something of a loss of toughness, although the strength is increased.

The effect of varying the temperatures and times of the age-hardeningtreatment, this for a preferred temperature of austenite-conditioningtreatment of 1700 F. for 1 hour and a generally preferred refrigerationtreatment of some 100 F. for 6 hours is shown by the mechanicalproperties of Heat 31562 as given in Table V below, all in transversedirection.

Table IV MECHANICAL PROPERTIES OF STEELS OF INVENTION FOR DIFFERINGCONDITIONS OF REFRIGERATION UNDER DIFFERING AUSTENITE-CONDITIONING BUTCONSTANT HARDENING CONDITIONS (975 F. 1 HR.)

Refrigeration Ult. Ten. 0.2% Y.S. Percent Sample Str. p.s.i. p.s.i. E1.in 2 Hard. RA Allison Parameter Temp. Time, F. Hrs.

B"Conditioned at 1,350 F. for 1% hours DConditioned at 1,550 F. for 1%hours GCondltioned at 1,750 F. for 1% hours Table V MECHANICALPROPERTIES OF STEEL OF INVENTION FOR DIFFERING CONDITIONS OI ACE-HAUSTENITE-CONDITIONING (1,700 F. 1 HR.) AND REFRIGERATION (-100 F FOfi 6I I O T l I: S AT CONSTANT Hardening Sample Ult. Tens. 0.2% Y.S. PercentHard Allison Parameter Str.p.s.1. p.s.i. E1. in 2 Bo Temp. F. Time, Hrs.

900 2% 229,200 210, 000 5 4s 4 900 5% 242, 500 222, 000 o 49 5 4 900 5/1 245, 200 224, 900 5 49. 5 39 43 39 55 975 1 234, 700 215, 000 5 48.554 55 o4 63 975 4 229,000 214,900 6 48.5 .51 44 '25 '57 975 4 230,800213, 300 4 41.5 .45 55 39 42 975 7 233, 400 213, 800 6 4s. 54 70 66 521, 050 192,200 177, 500 10 4s 1. 39 1. 23 1. 55 1I 3s With a preferredconditioning temperature of some 1700 F. for 1 hour followed by aparticularly preferred refrigeration treatment at 100 F. for 8 hrs, anexcellent combination of strength and toughness is had by finallyhardening th emetal at 950 F. for 1 hour. Moreover, by decreasing thetemperature of the age-hardening treatment to 900 F. and prolonging thetime for 8 hours, both the ultimate tensile strength and the yieldstrengh of the metal are increased without, however, any significantloss of toughness, the tensile and yield strengths then respectivelyamounting to some 260,000 p.s.i., and 241,000 p.s.i., with toughness ofabout .55 Allison Parameter, as contrasted with ultimate tensile andyield strengths of 232,000 p.s.i. and 216,000 p.s.i., at like toughness,lfOI' the 950 F. age-hardening treatment.

In the cold-rolled condition, this for a reduction on the order of some60%, my steel possesses great strength in the direction of rolling andin the transverse direction, as well. Although toughness is greatest inthe longitudinal direction, yet the toughness in the transversedirection is substantial.

The eifect of hardening temperatures and times of treatment on themechanical properties of cold-rolled sheet of Heat 31562 (analysis givenin Table 1(a) above), both in longitudinal direction and in transversedirection, is illustrated below in Table VI:

Parameter toughness ranging from .98 to 2.06. The transverse directionsample, having an ultimate tensile strength of 283,500 p.s.i., in likehardened condition, has an Allison Parameter figure ranging from .02 to.11.

With hardening conducted at the higher temperature of 1050 F. for 1 to5% hours, however, the toughness in transverse direction is subsantiallyincreased, this amounting to some .10 to .40 Allison Parameter, althoughthere is some loss in the ultimate tensile strength. The toughness inlongitudinal direction also falls considerably, nevertheless this stillis excellent, this ranging from some .58 to .89 Allison Parameter.

The ultimate tensile strength of the steels hardened at the highertemperature is to some extent sacrificed to gain the improved toughnessin transverse direction, this strength amounting to some 224,200 to244,400 p.s.i. in longitudinal direction and 233,500 to 237,000 p.s.i.in transverse direction for the steels hardened at 1050" F., as against270,000 to 278,000 p.s.i. in longitudinal direction and 283,500 to287,600 p.s.i. in transverse direction for steels hardened at the 900 F.temperature.

For a balance between strength and toughness in the longitudinal andtransverse directions the higher agehardening temperatures arepreferred. But for maximum toughness in longitudinal direction, withgreat strength, it

Table VI EFFECT OF VARIABLE HARDENIN G TREATMENT ON COLD-ROLLED SHEET OFHEAT 31562 Hardening Sample Ult. Tens. 0.2% Y.S. Percent Hard AllisonParameter Str. p.s.i. p.s.i. E1. in 2 R Temp. F. Time, Hrs.

Mechanical PropertiesLongitudinal Direction 000 2% 270, 000 261,200 251.5 2. 20 2. 39 1.67 1. 32 900 5% 271,000 260, 000 2 51.5 2. 21 2.37 1. 61 1. 63 900 7 278, 000 275,000 2 52. 5 98 1.10 l. 14 2.06 975 1275,000 263, 200 l. 5 51 2. 57 2. 27 2. 79 2. 975 4 253, 400 242, 200 22. 08 2. 08 2. 67 2. 02 975 4 252, 000 245, 200 2 50 1. 42 1. 39 2.69 1. 02 975 7 268, 200 255,000 50. 5 2. 05 1. 81 2. 77 2, 48 1, 050 l244, 400 233, 000 2 48. 5 1, 050 2% 227, 100 220, 000 4. 5 47. 5 58 1,050 5% 224, 200 217, 400 4. 5 47 89 .81

Mechanical Properties-Transverse Direction From the informationpresented in Table VI it is noted that in longitudinal direction thesample hardened at a temperature of 900 F. for the time of 7 hours hasan is the lower hardening temperature, a hardening temperature of 900 to975 F., that is preferred.

Thus it will be seen that I provide in my invention ultimate tensilestrength of 278,000 p.s.i., with Allison a semiausteniticprecipitation-hardenable chromimumnickel-aluminum stainless steel inwhich the various objects set forth above, together with many advantagesare successfully achieved.

The steel of my invention is of such composition balance that it may beshipped from the mill in cold weather without fear of prematurehardening. And when received by the customer-fabricator, is soft andductile and readily lends itself to forming as by pressing, bending,shrinking, stretching, and the like; readily machined as by sawing,cutting, tapping, threading, etc.; and readily fabricated as byriveting, welding, brazing and other known fabricating operations. Thesteel and various fabricated articles and products then are hardened bysimple heattreatment at comparatively low heat-treating temperatures toachieve great strength and toughness. Both strength and toughness arehad in working direction of the metal as well as in transversedirection. The metal is resistant to tearing.

I also provide a method of heat-treating my steel in order to achievethe surprising combination of strength and toughness in longitudinaldirection of working and in the transverse direction. The method issimple, direct and effective.

Since many embodiments of my invention may occur to those skilled in theart to which the invention relates, and many variations may be made inthe embodiments herein disclosed, it will be understood that all subjectmatter described herein is illustrative and is not to be taken aslimitative.

Having described my invention, I claim:

1. A semi-austenitic, precipitation-hardenable chromium-nickel-aluminumstainless steel essentially consisting of about 7.0% to 18.0% chromium,about 6.0% to 12.0% nickel, about .5% to 2.5% aluminum, manganese andsilicon each not exceeding about 1.0%, with a carbon content notexceeding .05%, a phosphorus content not exceeding about .040%, asulphur content not exceeding 0.010%, a nitrogen content not exceeding.05%, molybdenum up to about 8.0%, and remainder essentially iron.

2. A semi-austenitic, precipitation-hardenable chromium-nickel-aluminumstainless steel essentially consisting of about 7.0% to 18.0% chromium,about 6.0% to 12.0% nickel, about .5% to 2.5% aluminum, manganese andsilicon each not exceeding about 1.0%, about .002% to .050% canbon, aphosphorus content not exceeding .040%, a sulphur content not exceeding0.010%, a nitrogen content not exceeding .05 molybedenum up to about8.0%, titanium up to about .10%, boron up to about .003%, and remanideressentially iron.

3. A semi-austenitic, precipitation-hardenable chromium-nickel-aluminumstainless steel essentially consisting of about 7.0% to 18.0% chromium,about 6.0% to 12.0% nickel, about .5% to 2.5% aluminum, manganese andsilicon each not exceeding about 1.0%, with a carbon content notexceeding .05%, a phosphorus content not exceeding about .040%, asulphur content not exceeding 0.015%, a nitrogen content not exceeding0.01%, molybdenum up to about 8.0%, titanium up to about .10%, boron upto about .003%, and remainder essentially iron.

4. A precipitation-hardenable chromium-nickel-aluminum stainless steelessentially consisting of about 13.0% to 15.0% chromium, about 7.5% to9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 3.0%molybdenum, manganese and silicon each not exceeding about 1.0%, carbonnot exceeding .05%, phosphorus not exceeding .040%, sulphur notexceeding about 0.010%, nitrogen not exceeding .05%, and remainderessentially 11011.

5. A precipitation-hardenable chromium-nickel-aluminum stainless steelessentially consisting of about 14.2% to 14.7% chromium, about 8.1% to8.6% nickel, about 2.0% to 2.5% molybdenum, about 1.05% to 1.30%aluminum, manganese and silicon each not exceeding about .7%, carbon notexceeding .05%, phosphorus not exceeding about .040%, sulphur notexceeding about .005%,

nitrogen not exceeding .05%, and remainder essentially iron.

6. Precipitation-hardenable chromium-nickel-aluminum stainless steelplate, sheet, strip, bars, rods, wire and like products which inhardened condition is strong and tough, which products essentiallyconsist of about 13.0% to 15.0% chromium, about 7.5% to 9.5% nickel,about 2.0% to 3.0% molybdenum, about .75% to 1.50% aluminum, manganeseand silicon each not exceeding 1.0%, with carbon .002% to .050%,phosphorus not exceeding about .040%, sulphur not exceeding about .007%,nitrogen not exceeding .05%, and remainder essentially iron.

7. Precipitation hardened chromium-nickel-aluminum stainless steelessentially consisting of about 13.0%' to 15.0% chromium, about 7.5% to9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 3.0%molybdenum, manganese and silicon each not exceeding about 1.0%, about.02% to .045% carbon, phosphorus not exceding about 0.040%, sulphur notexceeding 0.015%, nitrogen not exceeding 0.05%, and remainderessentially iron.

8. Precipitation-hardenable chrimium-nickel-aluminum stainless steelessentially consisting of about 13.0% to 15.0% chromium, about 7.5% to9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 3.0%molybdenum, manganese and silicon each not exceeding about 1.0%, carbonnot exceeding .05%, phosphorus not exceeding about 0.040%, sulphur notexceeding about 0.015%, nitrogen not exceeding .01%, and remainderessentially iron.

9. Precipitation hardened chromium-nickel-aluminum stainless steelproducts having a tensile strength exceeding 225,000 p.s.i. incombination with a toughness exceeding .5 Allison Parameter, saidproducts essentially consisting of about 7.0% to 18.0% chromium, about6.0% to 12.0% nickel, about .5 to 2.5% aluminum, with carbon .002% to.05%, manganese and silicon each not exceeding 1.0%, phosphorus notexceeding about 0.040%, sulphur not exceeding about .007%, nitrogen notexceeding .05%, molybdenum up to 8.0%, and remainder essentially iron.

10. A precipitation-hardenable chromium-nickel-aluminum stainless steelessentially consisting of about 14% to 15% chromium, about 8% to 9%nickel, about 1% aluminum, about 2% to 3% molybdenum, manganese andsilicon each not exceeding about .7%, carbon not exceeding about .04%,sulphur not exceeding about .008%, nitrogen not exceeding about .04%,and remainder essentially iron.

11. Precipitation hardenable chromium-nickel-aluminum stainless steelplate, sheet, strip, bars, rods, wire and like products essentiallyconsisting of about 14% to 15 chromium, about 8% to 9% nickel, about 1%aluminum, about 2% to 3% molybdenum, manganese and silicon each notexceeding about .7%, with carbon not exceeding about .04%, sulphur notexceeding about .008% nitrogen not exceeding about .04%, and remainderessentially iron.

12. A precipit-ation-hardenable chromium-nickel aluminum stainless steelessentially consisting of about 7.0% to 18.0% chromium, about 6.0% to12.0% nickel, about .5% to 2.5% aluminum, carbon not exceeding .05%,sulphur not exceeding .005%, nitrogen not exceeding .05 and remainderessentially iron.

13. A precipitation-hardenable chromium-nickel-aluminum stainless steelessentially consisting of about 10.25% to 15.0% chromium, about 7.5% to9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 6.5%molybdenum, carbon not exceeding .05%, sulphur not exceeding .010%,nitrogen not exceeding .05%, and remainder essentially iron.

14. A precipitation-hardenable chromium-nickel aluminum stainless steelessentially consisting of about 10.25 to 15.0% chromium, about 7.5% to9.5% nickel, about .75% to 1.50% aluminum, about 2.0% to 6.5%molybdenum, Carbon not exceeding .05%, sulphur not exceed- 1 7 ing .015nitrogen not exceeding .01%, and remainder essentially iron.

15. In the production of precipitation-hardened chromium-nickel-aluminumstainless steel of great strength in combination with toughness, themethod which comprises providing a steel essentially consisting of about14% to 15% chromium, about 8% to 9% nickel, about 1% aluminum, about 2%to 3% molybdenum, carbon not exceeding about .04%, sulphur not exceeding008%, nitrogen not exceeding about .04% and remainder essentially iron;austeni-te-conditioning the steel at a temperature of about 1700 F. forone hour or more; transforming the same by refrigerating at atemperature of about -100 F.;

and precipitation-hardening by reheating at a temperature of about 900to 1050 F.

References Cited by the Examiner UNITED STATES PATENTS 2,505,764 5/1950Goller 75-124 2,958,618 11/1960 Allen 75-124 3,117,861 1/1964 Linnert etal 75124 3,131,055 4/1964 Behar 75-124 10 3,151,978 10/1964 Perry et a1.75-124 DAVID L. RECK, Primary Examiner.

H. W. TARRING, Examiner.

1. A SEMI-AUSTENITIC, PRECIPITATION-HARDENABLE CHROMIUM-NICKEL-ALUMINUMSTAINLESS STEEL ESSENTIALLY CONSISTING OF ABOUT 7.0% TO 18.0% CHROMIUM,ABOUT 6.0% TO 12.0% NICKEL, ABOUT .5% TO 2.5% ALUMINUM, MANGANESE ANDSILICON EACH NOT EXCEEDING ABOUT 1.0%, WITH A CARBON CONTENT NOTEXCEEDING .05%, A PHOSPHORUS CONTENT NOT EXCEEDING ABOUT .040%, ASULPHUR CONTENT NOT EXCEEDING 0.010%, A NITROGEN CONTENT NOT EXCEEDING.05%, MOLYBDENUM UP TO ABOUT 8.0%, AND REMAINDER ESSENTIALLY IRON.